What is a Cnidarian?

Transcription

1 Invertebrate What is a Cnidarian? 9000 species of jellyfishes, corals, sea anemones, hydras Mostly marine animals Radially symmetrical One body opening Two layers of cells organized into tissues with specific functions: ectoderm and endoderm 1

2 Origin of Sponges and Cnidarians Sponge fossil evidence: Precambrian (650 mya) Evolved from a group of flagellated protists that resemble the collar cells of sponges Cnidarians fossil evidence: Precambrian (630 mya) The larval form of cnidarians resembles protists Polyp or Medusa Polyp: the sessile form of a cnidarian its mouth is surrounded by tentacles ex: sea anemones, corals, hydras Medusa: the free-swimming motile form of a cnidarian umbrella-shaped floating body mouth is on the underside ex: jellyfishes 2

3 Body Structure Tentacles: a ring of flexible, tube-like structures surround the mouth Vary in length Used to capture food Cnidae: Stinging cells that contain nematocysts Located at the tips of the tentacles Stinging cells discharge nematocysts that capture or paralyze prey May contain toxic substances Discharged in response to touch or chemicals in the environment Body Structure Mouth: Simple gut Single opening for incoming and outgoing particles Tentacles bring food into the mouth for digestion 3

4 Body Structure Gastrovascular Cavity: endoderm that is adapted for digestion release enzymes undigested materials are ejected back out through the mouth Body Structure Bud: all cnidarians can reproduce both sexually and asexually a polyp reproduces asexually by budding a bud is a clone of its parent genetically identical 4

5 Comparing Body Structures of Cnidarians Respiration: Life functions since the cnidarian body is only two cell layers thin, every cell is exposed to water oxygen dissolved in water is able to diffused directly into body cells Excretion: carbon dioxide and other wastes can move out a cnidarians' body cells directly into the surrounding water 5

6 Reproduction Sexual: egg and sperm Sexual reproduction usually occurs in the medusa stage Asexual: budding Asexual reproduction can occur in either the polyp or medusa stage Life Cycle 6

7 Nervous System Simple nervous system: no brain nerve net that conducts impulses to and from all parts of the body cause muscle-like contractions in the two cell layers (i.e. tentacles) Class Hydrozoa Open gastrovascular cavity; no internal divisions (A) Hydroids Ex. Hydra Branching polyp colonies formed by budding Found attached to pilings and shells 7

8 Class Hydrozoa (B) Siphonophores Ex. Portuguese man-of-war (Physalia) Include floating colonies that drift on the ocean surface Each individual has a function that helps the entire colony One forms a large, blue, gas-filled float Others are responsible for reproduction, feeding Class Scyphozoa Gastrovascular cavity has four internal divisions Locomotion: Muscle-like cells in the outer cell layer that contracts the bell to propel the animal through the water Ex. Jellyfishes Can be found everywhere in the oceans (arctic to tropical water) Have been seen at depths of more than 3000 meter 8

9 Only polyp form Class Anthozoa Have many incomplete divisions in their gastrovascular cavities Ex. Sea anemones (thought to live for centuries), corals (colonies of polyps) Corals secrete protective calcium carbonate shelters around their soft bodies Provide food and shelter for many other marine species When a coral polyp dies, its shelter is left behind (adds to coral reef s structure) A coral polyp extends its tentacles to feed Symbiotic relationship with zooxanthellae (photosynthetic protists) Zooxanthellae produce oxygen and food that the corals use, while using carbon dioxide and waste materials produced by the corals Zooxsnthellae are responsible for the bright colors found in coral reefs Class Cubozoa Ex. Box jellyfish Square in shape; four evenly spaced tentacles Well-developed eyes for sensing light 9